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Conventional methods for peripheral nerve or plexus blockade have involved the identification of surface anatomic landmarks. Such landmarks serve as an approximate starting point for a search for the targeted nerve or nerves by needle exploration. The objective of needle exploration is to reach a finite endpoint that indicates the tip of the needle is sufficiently close to the targeted nerve or nerve plexuses. Two distinct types of endpoint exist.

  1. 1. An anatomic endpoint based on encountering anatomic relations to the targeted nerve or nerves. Examples of blocks that make use of anatomic endpoints include field block, transarterial techniques, or ultrasonographic guidance.

  2. 2. A functional endpoint based on a nerve response to mechanical or electrical stimulation. The main types of functional endpoints used clinically are either sensory response to mechanical stimulation of the nerve (mechanical paresthesia) or a motor response to electrical stimulation.

Designated anatomic landmarks are limited because they vary from patient to patient and do not always correlate with the location of the underlying nerve or plexus. In addition, traditional landmark measurements are sometimes complicated, requiring linear measurements with a ruler, bisecting lines, and they are not always normalized to patient size or body habitus. For many blocks, accepted descriptions of the technique include insertion of the block needle at a certain distance from a designated palpable landmark, without regard to patient size. Consequently, with many techniques, dexterity and delicate proprioception are often required for successful block performance.

Techniques such as ultrasonography or other imaging techniques or percutaneous localization utilizing transcutaneous electrical stimulation help to decrease needle exploration. Transcutaneous electrical stimulation, in contrast to an imaging technique such as ultrasonography, utilizes a functional neural response, either motor or sensory. Prelocalization of the nerve prior to needle insertion serves to decrease the amount of invasive search with the needle, increasing patient comfort while decreasing the potential for complications. The purpose of this chapter is to discuss how transcutaneous electrical stimulation helps to localize the underlying nerve or plexus through the skin, in a noninvasive manner before the needle is introduced transcutaneously. The chapter will necessarily discuss basic elements of nerve stimulation; however, for more in-depth coverage of principles of nerve stimulation and nerve stimulators, the reader is referred to Chapters 5 (Electrophysiology of Nerve Stimulation) and 17 (Equipment for Peripheral Nerve Blocks).

Elicitation of a paresthesia is an all-or-nothing phenomenon, ie, the needle either contacts the nerve or it does not. By contrast, use of electrical nerve stimulation yields graded information, which may be useful at a distance from the targeted nerve. Furthermore, visual cues of motor responses from untargeted nerves allow for redirection of the needle. This concept has been extended to the use of transcutaneous electrical stimulation to yield visual cues and motor responses, noninvasively, through the intact overlying skin.

Transcutaneous electrical stimulation to elicit a motor response has been used to assist in determining the optimal entry point ...

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